Electronic control systems are particularly well suited for applications in which operating conditions or parameters change. Electronic transmission controllers, for example, are becoming more prevalent because of their suitability for handling the complex tasks required for the shifting of a transmission between gear ratios.
Usually, electronic solenoids are used for the engagement/disengagement of the transmission's clutches. The engagement of a hydraulic clutch consists of two stages: the fill mode and the pressure modulation mode. In the fill mode, the clutch volume is filled with hydraulic fluid. In the pressure modulation mode, the pressure within the clutch volume is modulated (increased) to a pressure level to ensure proper and full engagement of the clutch. To actuate the clutch, the solenoid is therefore, first energized to begin filling the clutch.
After a predetermined time, the level of energization may be dropped, for example, by twenty-five percent, to avoid pressure spikes when end-of-fill is reached.
When the clutch is filled, the current applied to the solenoid is modulated (typically, in an increasing linear ramp function) to continue the flow of hydraulic fluid to the clutch and, thereby, increase the pressure to a level sufficient to properly engage the clutch.
Typically, a timing strategy has been used to determine when the clutch has reached the end of fill condition. In this situation, the solenoid's coil would be energized and the clutch would begin to fill with hydraulic fluid. After a predetermined time period, the transmission controller would begin to modulate current, in an effort to fully engage the clutch.
This procedure has several limitations. For example, operating conditions change the actual time required to fill the clutch. Since pump flow is a function of engine speed, pump flow will vary with engine speed. Other factors (for example, other hydraulic systems being supplied by the pump) may also affect pump flow. As the pump flow varies, the time required to fill the clutch will also vary. Other operating conditions which affect the clutch fill times are present gear ratio, desired gear ratio, transmission load, and inclination of the vehicle.
Variations in the engine and operating characteristics of the transmission components can be expected over the life of the vehicle due to wear. This will also affect the clutch fill time.
Furthermore, the variations in the system components, including the clutches, due to manufacturing tolerances will also affect clutch fill time.
If the proper fill time is not known or accurately estimated, the clutch will be in a overfill or underfill condition when the controller attempts to modulate clutch pressure to fully engage the clutch.
Operation of the transmission by modulating the clutch pressure in a underfill or overfill condition will cause a "jerky" shift action and increase the rate at which wear and tear occurs.
In an attempt, to predict fill times, it is known to add sensors to the transmission controller. For example, U.S. Pat. No. 4,707,789 issued to Robert C. Downs et al., on Nov. 17, 1987, uses a transmission input speed sensor to detect underfill/overfill condition. The time delay used to estimate clutch fill is adjusted based upon the transmission input speed. However, transient changes, that is, changes in the operating conditions that the controller has not adapted to, will affect the shift quality. Furthermore, a transient condition will have a negative effect on the fill time for the next shift without the transient condition.
In another attempt to accurately predict the end of fill condition, it is known to add additional valves to the controller. One such system is shown in the Komatsu technical guide, "K-ATO1VIICS Komatsu-Advanced Transmission with Optimum Modulation Control". A flow sensing valve is used to sense a pressure differential. The spool of the flow sensing valve closes a switch in response to the pressure differential, thereby, signalling the end of fill condition. In still another attempt, hydraulic pressure is used to predict the end of fill condition. U.S. Pat. No. 4,942,787 issued to Takashi Aoki et al, on Jul. 24, 1990 discloses the use of a pressure detection switch for that purpose. However, the cost added by the additional components in both these systems, plus, the added manufacturing cost due to the increased complexity, make these systems undesirable.
Still another attempt to detect the end-of-fill condition of a transmission clutch is disclosed in U.S. Pat. No. 5,045,599 issued to Tony L. Marcott on Oct. 8, 1991 ('599). The apparatus disclosed in '599 uses the detection of a voltage spike generated in the coil current as a result of end-of-fill. The voltage spike is generated by an electromotive force applied to the spool in a control valve at end-of-fill. However, it has been discovered that for some applications, it is desirable to detect end-of-fill earlier than at the point in time when the voltage spike is generated.
The present invention is directed at overcoming one or more of the problems as set forth above.